When you think of a shark what distinctive features do you think of? Sharp teeth? A streamlined body? Distinctive fins? Sharks are a diverse group of fishes, and though there is no one way to describe all sharks, they all have some common characteristics which is what makes a shark a shark.
Scientists have created a logical system of classifying most living organisms on Earth using a system called taxonomy. Sharks fit into the system under the following organization:
How did scientists come up with this classification scheme? These fishes share certain characteristics. To be classified in Class Chondrichthyes, the fish has to have a skeleton made of cartilage, not bone, have a true upper and lower jaw, and nostrils below their head. Subclass Elasmobranchii fish have multiple paired gill openings on the side of their heads. There are more specific characterizations that further define each family, genus and species within every order.
Shark taxonomy is a dynamic system, hardly agreed upon, as new specimens are discovered, needing cataloging and more in-depth analysis is completed on groups previously classified.
Most sharks are considered to be fusiform shaped, spindle/torpedo shaped, tapering at each end creating a streamlined body. Though there are variations on this body type, it is the most common shark body type and the one we usually first think of. Skates and rays, who are related to sharks, usually differ in they are flattened or disc shaped.
Sharks have five rigid fins which they can't fold down against their bodies (unlike bony fishes). Unlike bones in fish fins, shark fins are supported by soft, unsegmented rays of large collagen fibers called "ceratotrichia". The triangular dorsal fin atop the shark helps the shark with balance. Most species have two dorsal fins but some only have one. The tail fin of sharks is called the caudal fin is considered heterocercal, meaning the top and bottom lobes are not symmetrical and the vertebral column extends upwards into the top lobe. Sharks move their caudal fins side to side to propel themselves forward. The caudal and pectoral fins roughly correspond to human arms and legs. On the bottom side of the shark, there are two other fins, the pelvic fin and the anal fin. The pelvic fin is associated with claspers, the male sexual organ, and in some species of sharks the anal fin is absent.
A shark’s skin is made up of dermal denticles. These tiny tooth-like scales cover the shark, although they are absent on the underside of some species, and each has a sharp edged crown covered in enamel. Millions of these denticles cover the shark, making its skin feel rough to the touch. The denticles point backwards helping the shark swim efficiently and faster by reducing water resistance. As a shark grows it sheds and replacement scales grow through the skin. The dermal denticle in some species have evolved into specialized forms, like a dorsal fin spine that grows continuously and therefore can be used in age studies. The skin on females of some species is considerably thicker than males because the males may hold the females during copulation which may inflict serious bites.
Sharks have receptors that are sensitive to electric fields. These receptors are called ampullae of Lorenzini. The ampullae are canals filled with a kind of jelly on the shark’s head that are able to receive electric stimuli through the skin of the shark. The shark is able to ‘read’ minute changes in electric current in its environment through the ampullae. This is a sense every bit as developed and important to the shark as sight, hearing, smell, touch, and taste.
How does the shark use its electro-reception? When a fish or some other prey moves its muscles, there is a slight change in electric current. A fish that is wounded and struggling will produce a different amount of electricity. Sharks are able to sense this weak change in current through the water even before they might otherwise see or smell them, and before their prey can see the shark. All shark species use this electroreception ability to locate prey. Some species that enjoy bottom-dwelling fish or mollusks that bury themselves in the sand are able to locate these animals with their electro-sense.
Sharks also use their electro-sensors to help navigate the oceans. They are able to induce an electric current by their own movement across the earths electromagnetic fields, and this helps them to determine and stay on course.
Most species of sharks have five pairs of gill slits on each side of their head, but a few have six or seven pairs. The gills are crucial to a sharks breathing system. By opening its mouth, water is drawn into the shark. The open mouth gives the appearance of a smile in many sharks.
When a shark closes its mouth, the water is forced over its gills and the tiny blood vessels in shark gills absorb oxygen from the water. To aid water flowing over its gills most sharks swim forward all the time, called ram ventilation, and is characteristic of more active open ocean sharks. If these sharks are trapped or forced to stop swimming, they can suffocate. Some sharks can actively pump water over their gills, called buccal pumping, while resting on a sandy bottom or under a ledge, like the nurse shark.
A simple way to distinguish a shark from a skate or ray is the position of the gill slits: they are on the sides of the head in sharks, while below the head near the mouth on skates and rays.
A shark’s upper jaw is not fused to its cranium. This means that their jaws can partially protrude from their head, orientating their teeth outward, maximizing the amount of prey. Sharks can open their mouth very wide, sometimes to nearly 180 degrees, to catch their prey (try opening your mouth and see how wide you can do this!).
A shark’s teeth are larger versions of the denticles covering a shark’s body. Sharks are famous for their teeth, which they produce and shed throughout their life. They grow in rows and move forward on a conveyor belt-like system. The rate of tooth loss is very dependent on the individual species and time of year. A shark may produce, use, and shed as many as 6,000 teeth each year, and over a lifetime a shark may produce tens of thousands teeth! Shark’s teeth are so hard, covered in resilient enamel, they are resistant to erosion (unlike shark’s cartilaginous skeleton) which is why shark’s teeth are so numerous in the fossil record. Different sharks have different shaped teeth depending on what food they eat. Sharks which eat fish have pointed teeth for ‘catching’ their prey, those that sometimes eat seals or sea lions have razor sharp teeth for cutting out manageable cunks of prey, sharks that eat shellfish and crabs have flat crushing teeth, and filter feeding sharks have greatly reduced teeth that they don‘t use, instead they utilize dermal denticles lining the gill plates and pharynx, a modification of gill rakers, long slender filaments that strain plankton from the water.
The body tissues of a shark are denser and heavier than water, so naturally a shark would slowly sink. To compensate for this sharks have a light, cartilaginous skeleton and a big oil-filled liver. Sharks lack any trace of a swim bladder (the gas-filled buoyancy-control organ found in many bony fishes). Instead they use their oil-filled liver to reduce their over-all density and provide them with some ability to ascend and descend in water. An enormous liver dominates the shark body.
Fish such as tuna and mackerel have a skeleton made of bone (just like humans), but a shark skeleton is made entirely of cartilage (like human noses and ears). Cartilage is lighter and more flexible than bone, thus helping the shark to both stay neutrally buoyant and more easily flex its body to swim and navigate. At strategic points in a shark’s body, hexagonal plates made up calcium salts add strength to the cartilage skeleton, such as in the jaws and the backbone.
Sharks have two kinds of muscles. The first is a thin layer of red muscle, a slow contracting muscle layer used for cruising, just under their skin. This red muscle requires an oxygen rich blood supply. Red muscle works by breaking down fat stored in a shark’s body.
The second type is white muscle, a fast contracting muscle layer for bursts of speed, is found under the red muscle. White muscle doesn’t require an oxygen-rich blood supply and works by using energy from the breakdown of glycogen (sugars).
Most sharks pump large amounts of oxygenated blood around their body, feeding their musckes and producing heat. That heat is usually lost when the warm blood flows through the thin walls of the vessels in the gills to pick up oxygen from the colder sea water. These sharks are considered cold-blooded or ectothermic (their body temperature is the same as the surrounding water). Some sharks, like the Mako, White, Porbeagle, and Thresher, are warm-blooded, or endothermic, meaning they can maintain a core body temperature above that of the outside water temperature. These sharks have developed a ‘rete mirable’, a fine network of capillaries where cold oxygen-rich blood runs right next to, but in the opposite direction of, warm poorly oxygenated blood, exchanging heat that is carried back to the muscles, not lost to the sea. This adaptation makes them more efficient and faster growing than similar ectothermic sharks. Salmon shark’s can sometimes elevate their body temperature about 21° C above the cold surrounding water temperature.
Unlike most vertebrates who simply excrete nitrogenous waste, sharks retain high concentrations of urea and trimethylamine oxide in their body tissue because they are important in regulating a shark’s internal salt-to-water balance.
